This invention relates to an improved differential stage particularly for active filters.
As is known, active filters utilize differential stages which are required to simultaneously have a high input dynamic range and the ability to change the stage transconductance (i.e., the ratio of output current to input differential voltage). However, it has been impossible, heretofore, to meet both of these requirements with prior circuits, and more generally, it has not been possible to achieve a high input dynamic range in circuits featuring a variable transconductance, while circuits having a higher input dynamic range could not provide variability of transconductance.
In particular, prior circuits may be, among other things, referred to two basic schemes, respectively as shown in FIGS. 1 and 3.
In the circuit of FIG. 1, including two transistors whose emitters are connected together and directly to a source of constant current, a transconductance is obtained which is directly proportional to the generator current.
Thus, defining ##EQU1## if Q.sub.1 =Q.sub.2, therefore ##EQU2## Hence, the stage transconductance is proportional to the imposed current I.sub.o and can be varied by varying the latter.
On the other hand, with this circuit, the maximum input dynamic range (i.e., the maximum differential voltage that may be applied between the bases of the two transistors) is restricted to approximately 3 V.sub.t.
In fact, a large signal analysis of the circuit (see, for example, Gray, Meyer "Analysis and Design of Analog Integrated Circuits", John Wiley & Sons, 1977, p. 158 seq.) would give ##EQU3## the behavior whereof versus the input differential voltage is shown in FIG. 2.
In the circuit of FIG. 3, wherein to increase the input dynamic range, two resistors have been connected serially to the emitter electrodes of the two transistors, while an improvement is achieved in the input dynamic range, variability of transconductance as a function of the imposed current goes lost.
In that case, in fact ##EQU4## while the input dynamic range is linear with the current I.sub.o EQU v.sub.dMAX .perspectiveto.2I.sub.o R.sub.E